As the processing speed increases in sawmill machinery, wood pieces tend to bounce back from bumpers and alignment gates and are not always presented to the sawmill equipment in an ideal position. This inherent disadvantage with the handling of wood pieces is particularly apparent in wood cants or flitches. Wood cants have irregular and non-parallel sides which make them difficult to align along the longitudinal axis of an infeed conveyor for example. Consequently, increasing the processing speed of machinery often results in less recovery.
In the present description, the words; wood piece, cant, flitch and board are used interchangeably to designate a lengthwise strip of wood cut from a tree trunk.
In view of increasing both the processing speed and recovery, lineal scanners and computers have been developed to precisely measure the dimensions and the position of a wood board on a conveyor. These scanners and computers generate three-dimensional images of the cant, and calculate a sawing solution that represents the highest value combination of products which can be produced from the cant.
Similarly, sawmill edgers have been developed to operate with lineal scanners and computers. These edgers have a saw box that is adjustable about a vertical axis, and saw blades that are movable sideways along the arbor. The positions of the saw blades are continuously adjusted to track the realtime position and alignment of a wood board being fed there through and to follow the optimized cutting profile defined by the computer.
Examples of optimized edgers available in the prior art are disclosed in the following documents;    U.S. Pat. No. 4,239,072 issued Dec. 16, 1980 to H. Meriläinen;    U.S. Pat. No. 5,722,474 issued Mar. 3, 1998 to C. Raybon et al.;    U.S. Pat. No. 5,816,302 issued Oct. 6, 1998 to W. R. Newnes;    U.S. Pat. No. 5,884,682 issued Mar. 23, 1999 to J. B. Kennedy et al.;    U.S. Pat. No. 5,946,995 issued Sep. 7, 1999 to S. W. Michell et al.;    U.S. Pat. No. 6,178,858 issued Jan. 30, 2001 to M. P. Knerr et al.;    U.S. Pat. No. 6,202,526 issued Mar. 20, 2001 to M. Dockter et al.
It will be appreciated that in a continuous wood edging process, the cants to be trimmed must be located precisely such that the saw blades can track the optimized cut lines in one cant and reposition quickly to track the optimized cut lines in a next cant. It has been found, however, that when the leading edge of a saw blade is made to focus on the leading edge of a cant approaching at high speed, there is a certain amount of wandering of the saw blade before it is set to track the optimized cut line. The saw blade enters the leading edge of the cant in a milling mode rather than a sawing mode, thereby increasing the kerf width at the leading edge of the cant. Similarly, when the optimized cut line stops at the trailing edge of the cant, the saw blade stops tracking the optimized cut line before it has completely exited the cant, causing an aftercut and also increasing the kerf width at the trailing edge of the cant.
In the machines of the prior art, several methods are used to locate the leading and trailing edges of a cant to control the tracking of optimized cut lines. For example, the machine described in U.S. Pat. No. 4,239,072 uses several measuring gates on the infeed side of the cutter heads to determine the position of the cant relative to the cutter heads and to adjust the cutter heads prior to entering into the cant. The position of the cant is measured relative to a feeding line. The cutter heads are correspondingly positioned on both sides of the feeding line, and the tracking of the optimized cut lines starts as the cant passes through the edger. The cutter heads are inclined in relation to each other in such a manner that the cutter heads are closer to each other at their cutting side than at the exit side to prevent aftercut.
The machine disclosed in U.S. Pat. No. 5,722,474 uses photodetectors to detect the location of a cant relative to a reference point. Then the movement of the saw blades is correlated by computer with the longitudinal movement of the cant past the reference point.
The machine described in U.S. Pat. No. 5,884,682 uses another approach. The machine uses mechanical positioning devices to position the cant and to present it tangentially to the saw blades.
As it was explained, there are drawbacks in adjusting the saw blades to follow optimized cut lines which start at the leading edge of the wood board and end at the trailing edge of the board. As such, it may be appreciated that there continues to be a need for a new and improved method to operate a board edger to prevent these surface defects. There is also a need for a better board edger in which the saw blades are shifted with greater speed and precision.